This invention generally relates to the field of fuel additive compositions and, more specifically, to fuel additive compositions capable of increasing the efficiency of combustion systems i.e. continuous combustion systems (boilers, furnaces etc.) and internal combustion systems (vehicles etc.) thereby increasing fuel economy, decreasing the amount of harmful pollutants formed in the combustion process, reducing the corrosive effects of fuels, and reducing engine noise and roughness.
In recent years, there has been an increased awareness of the need for greater fuel efficiency and maximum pollution control from combustion of fossil fuels. Fuel additives have long been employed to provide a variety of functions in fuels intended for use in combustion systems, and have demonstrated varying degrees of effectiveness. For example, Kaspaul describes in U.S. Pat. No. 4,244,703 the use of diamines, especially tertiary diamines, with alcohols as fuel additives to primarily improve the fuel economy of internal combustion engines. Similarly, Metcalf describes in GB 0990797 the use of an admixture comprising formaldehyde or polymeric formaldehyde, a combined acrylic ester and acrylic resin solution, methylene glycol dimethyl ether, propanediamine, and butyl-paraphenylene diamine in a carrier or solvent as a fuel additive primarily intended to improve the fuel economy of internal combustion engines. The fuel additives described by Knight in GB 2085468 comprising aliphatic amines and aliphatic alcohols serve as anti-misting additives for aviation fuels while GB 0870725 describes the use of N-alkyl substituted alkylene diamines as anti-icing agents. Only a few of those compositions either claimed to or actually do improve combustion efficiency, but none have proved completely successful. Furthermore, none of the known compositions have been able to successfully fill the need for fuel additives which, when added to fuels, provide greater fuel efficiency, maximum pollution control, and reduction of the corrosive effects of fuels on combustion systems.
The need to reduce the amount of harmful pollutants formed in the combustion process is great. On complete combustion, hydrocarbons produce carbon dioxide and water vapor. However, in most combustion systems the reactions are incomplete, resulting in unburned hydrocarbons and carbon monoxide formation which constitutes a health hazard. Moreover, particulates may be emitted as unburned carbon in the form of soot. Sulphur (S), the major fuel impurity is oxidized to form sulphur dioxide (SO.sub.2) and some is further oxidized to sulphur trioxide (SO.sub.3). Furthermore, in the high temperature zones of the combustion system, atmospheric and fuel bonded nitrogen is oxidized to nitrogen oxides, mainly nitrogen oxide (NO) and nitrogen dioxide (NO.sub.2). All these oxides are poisonous or corrosive. When oxidized in the combustion zone, nitrogen and sulphur form NO, NO.sub.2, SO.sub.2 and SO.sub.3. NO.sub.2 and SO.sub.3 are the most harmful of these oxides.
Pollutants also arise due to incomplete combustion of the fuel, these being particulates, hydrocarbons and some carbon monoxide. The desired goal of reducing the amounts of both groups of pollutants is very difficult to achieve due to the mutually contradictory nature of the formation of these pollutants. Nitrogen and sulphur oxides require a depletion of oxygen or, more specifically atomic oxygen, to prevent further oxidation to the higher more deleterious oxides; and the particulates require an abundance of oxygen to enable complete oxidation of the unburned fuel.
It is believed that anything which can mop up atomic oxygen will reduce formation of the higher oxides of nitrogen and sulphur. It is well known that atomic oxygen is responsible for the initial oxidation of SO.sub.2 to SO.sub.3 within the reaction zone. Therefore any reduction in atomic oxygen will lead to a reduction of SO.sub.3 and NO.sub.2.
The oxides produced during combustion have a deleterious effect on biological systems and contribute greatly to general atmospheric pollution. For example, carbon monoxide causes headaches, nausea, dizziness, muscular depression, and death due to chemical anoxemia. Formaldehyde, a carcinogen, causes irritation to the eye and upper respiratory tract, and gastrointestinal upsets with kidney damage. Nitrogen oxides cause bronchial irritation, dizziness, and headache. Sulphur oxides cause irritation to mucous membranes of the eyes and throat, and severe irritation to the lungs.
In addition to contributing to air pollution, combustion by-products, especially sulphur (S), sodium (Na) and vanadium (V), are responsible for most of the corrosion which is encountered in continuous combustion systems. These elements undergo various chemical changes in the flame, upstream of the corrosion susceptible surface.
During combustion, all the sulphur is oxidized to form either SO.sub.2 or SO.sub.3. The SO.sub.3 is of particular importance from the point of view of plant and engine corrosion. The SO.sub.3 combines with H.sub.2 O to form sulfuric acid, H.sub.2 SO.sub.4 in the gas stream and may condense out on the cooler surfaces (100.degree. C. to 200.degree. C.) of air heaters and economizers, causing severe corrosion of these parts. The formation of SO.sub.3 also causes high temperature corrosion.
SO.sub.3 formation most probably occurs via the reaction of SO.sub.2 with atomic oxygen. The oxygen atom being formed either by the thermal decomposition of excess oxygen, or the dissociation of excess oxygen molecules by collision with excited CO.sub.2. molecules which exists in the flame:
CO+O .fwdarw.CO.sub.2 *
CO.sub.2 *+O.sub.2 .fwdarw.CO.sub.2 +20
The residence time of bulk flue gases within a continuous combustion system is normally insufficient for the SO.sub.3 concentration to approach its equilibrium level, most of the SO.sub.3 present originating in the flame. The net result is that the steady state SO.sub.3 concentration in the flue gas is normally of the same order as, but slightly less than, that generated in the flame. Therefore, it is essential to reduce SO.sub.3 concentrations in the flame. To achieve this, excess oxygen concentrations must be minimized. However, reduction of oxygen also leads to incomplete combustion and particulate and smoke formation. To achieve this balance is extremely difficult in large continuous combustion systems and, therefore, a fuel additive which could manipulate the combustion reactions to reduce SO.sub.3 formation without incurring increased soot and particulate penalties would be highly desirable.
Compared with sulphur, the behavior of sodium and vanadium are more complex. The sodium in oil is mainly in the form of NaCl and is vaporized during combustion. Vanadium during combustion forms VO and VO.sub.2 and, depending on the oxygen level in the gas stream, forms higher oxides, the most harmful of which is vanadium pentoxide (V.sub.2 O.sub.5). V.sub.2 O.sub.5 reacts with NaCl and NaOH to form sodium vanadates. Sodium reacts with SO.sub.2 or SO.sub.3, and O.sub.2 to form Na.sub.2 SO.sub.4.
All these condensed compounds cause extensive corrosion and fouling of the combustion system. The degree of fouling and corrosion is dependent on a number of variables and occur to different extent at different locations in the combustion system.
One of the most important pollutants formed by oil combustion is oil-ash, which in the presence of SO.sub.3 forms complex, low melting point, vanadyl vanadates, for instance Na.sub.2 O.V.sub.2 O.sub.4 0.5V.sub.2 O.sub.5 and the comparatively rare 5-sodium-vanadyl 1.11-vanadate (5Na.sub.2 O.V.sub.2 O.sub.5 0.11V.sub.2 O.sub.5). Thus, high temperature corrosion can occur when the melting point of these substances are exceeded since most protective metal oxides are soluble in molten vanadium salts.
These observations have lead to a variety of proposals for minimizing corrosion. The known techniques have their advantages and disadvantages but none have been able to fill the need for fuel additives which are commercially viable and minimize corrosion without undesirable side effects. However, it is known that if SO.sub.3 formation could be suppressed, V.sub.2 O.sub.5 and other harmful by-products would be minimized inherently.
It will be appreciated that it is very difficult to establish the characteristics which are likely to enhance combustion of the fuel because of the very rapid and complex nature of the combustion process. Not surprisingly, numerous theories have been put forward for the combustion process, some of which conflict with one another.
It is convenient to split the combustion process into three distinct zones, namely a preheat zone, the true reaction zone and a recombination zone. With the majority of hydrocarbons, in the preheat zone degradation occurs and the fuel fragments leaving the zone will generally comprise mainly lower hydrocarbons, olefins and hydrogen. In the initial stages of the reaction zone the radical concentration will be very high and oxidation will proceed mainly to CO and OH. The mechanism by which CO is then converted into CO.sub.2 during combustion has been the subject of controversy for many years. However, it is believed that the nature of the species in the true reaction region is critical for the oxidation. In this region many species are competing for the available atomic oxygen, including CO, OH, NO and SO.sub.2. Compared with the many transient species present in the initial stages of a flame, the concentration of CO, NO and SO.sub.2 is large. CO and OH will readily react with oxygen radicals to form CO.sub.2 and H.sub.2 O and the oxidation of these can be complete in the initial stages of the flame. If initiation of reaction occurs near the beginning of the reaction zone this will allow the OH and CO species greater time to react with the available oxygen radicals. This will ensure that the duration of time spent by the species within the reaction zone is increased and therefore greater completion of the combustion reaction occurs.
From this theory it will be appreciated that if additives can be found which shorten the ignition delay this will, in turn, initiate early reaction thus allowing greater time of OH and CO to react. In doing so, OH and CO compete with SO.sub.2 and NO for the available atomic oxygen in the true reaction region.
The fuel additives of the present invention increase the operating efficiency of combustion systems by reducing the ignition delay of fuels and thereby improving the combustion characteristics of a system in which the given fuel is burned. The present additives initiate and quicken the ignition process thereby providing improvements in the combustion process resulting in reduced emissions of harmful pollutants, increased fuel economy, reduced corrosive effects on the system, and reduced engine noise and roughness in the case of internal combustion systems.